In the case where an image is formed by an ink-jet recording method or the like, there may be sometimes used an ink set comprising a light black ink for the purpose of obtaining a high quality image. For example, to a four-color ink set comprising a black ink, a cyan ink, a magenta ink and a yellow ink (or a six-color ink set further comprising a light cyan ink and a light magenta ink added thereto) may be added a light black ink to make a five-color ink set (or seven-color ink set). The light black ink has a lower carbon black concentration as compared with the black ink and is used for the purpose of improving the color reproducibility with respect to dark color such as shadow area or improving the gradation of gray.
However, since the light black ink (i.e., light black ink) has a low carbon black content, the resulting color hue is tinted rather than achromatic one which is originally necessary. In other words, the light black ink deviates from (a*, b*)=(0, 0) in the L*a*b* space of color difference representation method defined in CIE (Commission International de l'Eclairage). In some detail, the light black ink is tinted yellow. In order to modify the yellowing property of carbon black, it is necessary to use a colorant (e.g., pigment) complementary against yellow.
For example, U.S. Pat. No. 5,803,958 discloses a black pigment ink composition comprising a cyan pigment and a magenta pigment in addition to a carbon black. That is, it is described that the mixing of a cyan pigment and a magenta pigment achieves an effect similar to the effect in the case of adding a blue pigment and the use of this black ink composition as a light black ink composition makes it possible to improve the foregoing problem concerning yellowing property.
On the other hand, even when a black ink having a high carbon black content (i.e., dark black ink) is used, it is known that the resulting color hue is tinted red rather than achromatic one which is originally necessary. As a means for modifying such a reddening property, JP-A-64-180 discloses a method of adding a copper phthalocyanine pigment, for example.
However, no precise report on the relation between such yellowing and reddening properties and a carbon black content is known until now. Moreover, a method of modifying the yellowing property in the light black ink and the reddening property in the dark black ink has not even proposed in the art.
Separately, an ink-jet recording method using two or more black inks having different carbon black concentrations are also known and the addition of different types of dyes or pigments to a black ink having a high concentration and a black ink having a low concentration is also know (JP-A-11-48502). However, as the dyes or pigments to be added to these individual black inks, it is only described that black dyes are used singly or as a mixture, or a yellow, a magenta, and a cyan are used in combination. Thus, means of modifying the above yellowing property and reddening property at the same time is not even suggested.